I just finished The Last Witchfinder by James Morrow.

I've only read two books by Morrow (I wrote about the other one here), but I liked both of them and will keep seeking out other books by him. Most books today are easily categorizable and seem familiar when read, even if they are new and the author has come up with an unusual plot, but Morrow's books aren't and don't. He is a self-declared secular humanist, and his books reflect that with themes of reason vs. faith, and other philosophical issues. This makes him rare from two standpoints - having an explicit philosophy that informs his writing, and having an uncommon one as well.

The Last Witchfinder is a good example. In some ways, it is a straighforward historical novel set around the time of the Enlightenment in both Europe and North America. The protagonist's quest is to see the practice of "pricking", i.e. witchfinding, ended and this quest takes them from England to British North America and back and involves Newton and Ben Franklin. But in a weird twist, the story is narrated by another book, Newton's Principia Mathematica, which provides background information, as well as waging a war with the Malleus Maleficarum ("Hammer of Witches". It starts slow, but definitely picks up steam.